In one aspect, this invention relates to the separation of aromatic hydrocarbons from close-boiling alkanes (paraffins) by extractive distillation. In another aspect, this invention relates to the separation of cycloalkanes (naphthenes) from alkanes by extractive distillation. In a further aspect, this invention relates to the separation of alkenes (monoolefins) from alkanes by extractive distillation.
Extractive distillation is a well known technique for separating mixtures of components having a relative volatility close to unity (i.e., having nearly equal volatility and having nearly the same boiling point). It is difficult to separate the components of such mixtures by conventional fractional distillation. In extractive distillation, a solvent is introduced into a distillation column above the entry point of the feed mixture which is to be separated. The solvent affects the volatility of the higher boiling feed component(s) sufficiently to facilitate the separation of the various feed components by distillation and exits with the bottoms fraction, as has been described in the article entitled "Extractive Distillation Saves Energy" by Ian Sucksmith, Chemical Engineering, June 28, 1982, pages 91-95. Other literature sources on extractive distillation techniques include the "Handbook of Separation Techniques for Chemical Engineers" by Philip A. Schweitzer, McGraw-Hill Book Company, 1979, pages 1-135 to 1-143; Perry's Chemical Engineers Handbook, 6th Edition, McGraw-Hill Book Company, 1984, pages 13-53 to 13-57, and U.S. Pat. No. 4,921,581.
Even though the separation of various hydrocarbons, such as aromatics or cycloalkanes, from other close-boiling hydrocarbons, such as paraffins, by extractive distillation is known, there is an ever present need to develop novel solvent systems which exhibit advantages (such as higher selectivity) over known solvents in the extractive distillation of mixtures of close-boiling hydrocarbons. In particular, it is highly desirable to develop improved extractive distillation processes for separating aromatics or cycloalkanes or alkenes from close-boiling alkanes at high selectivity.